The present invention relates to a method for heat treating a metallic material and an apparatus therefor, in particular to a method and an apparatus for altering the microstructure of the metallic material by absorbing hydrogen in the metallic material and releasing the hydrogen from the hydrogenated metallic material so as to adjust their physical properties or pulverize the material.
Conventionally, a method for adjusting the physical properties of metallic materials (for example, improvement in the magnetic properties of rare earth (R)--Fe--B alloys by grain refining, or improvement in fatigue and creep resistance of Ti-based alloys by grain coarsening), is to apply an absorption/desorption cycle of hydrogen at a certain temperature. An apparatus, shown in FIG. 1, for performing such an operation has been examined in the past.
The heat treating apparatus 101 comprises: a heat treating furnace 102 for receiving a metallic material W and heating the metallic material W to a specific temperature; a hydrogen cylinder 104 connected to the furnace 102 via a gas supply path 103 for storing the hydrogen gas; an exhausting means 105 connected to the furnace 102 for exhausting the gaseous atmosphere from inside the furnace 102; an exhaust gas treatment means 106 for combusting the exhausted gas and discharging the combusted gas out of the apparatus.
In the above heat treating apparatus 101, the process comprises the steps of: charging the metallic material W into the furnace 102 and evacuating the furnace; absorbing hydrogen in the metallic material W by supplying hydrogen into the furnace 102 from the hydrogen cylinder 104 and holding the metallic material in the furnace 102 at an elevated temperature between 500.degree. to 1000.degree. C.; reducing the pressure inside the furnace 102 while holding the temperature of the furnace 102 at the above-noted temperature, thereby releasing the hydrogen from the metallic material W.
The apparatus is set up so that the hydrogen released from the metallic material W is withdrawn by the exhausting means 105, and, after being combusted by the subsequent exhaust gas treatment means 106, is expelled out of the apparatus.
In the above-described conventional method, there is a problem that the hydrogen for use in treating the metallic material W is consumed by being expelled out of the apparatus, therefore fresh hydrogen must be supplied for each treatment, resulting in a need to supply a huge quantity of hydrogen, and further resulting in a need for a hydrogen cylinder 104 of a large storage capacity.
As a counter measure for the problem, a consideration may be given to returning the hydrogen exhausted from the furnace 102 back into the hydrogen cylinder 104, for example.
However, even for this type of method, it is necessary to provide a liquefaction facility to convert the gasified hydrogen back to the liquid state, which requires a large sized processing facility, thus leading to a high cost of constructing a treatment facility. Therefore, recycling of hydrogen as described in the above manner is not an effective solution to the problem of the existing technology. This difficulty is made worse in the case of a multiple heat treating facility, because a hydrogen supply system must be provided for each heat treating furnace.
Further, a possibility can be considered that in using the recycled hydrogen gas, impurities which may have become included in the gas could affect the properties of the material being processed with the recycled gas.